Prevention of lens protein glycation by taurine

A Study of the Metabolic Pathways Affected by Gestational Diabetes Mellitus: Comparison with Type 2 Diabetes

BACKGROUND

Gestational diabetes mellitus (GDM) remains incompletely understood and increases the risk of developing Diabetes mellitus type 2 (DM2). Metabolomics provides insights etiology and pathogenesis of disease and discovery biomarkers for accurate detection. Nuclear magnetic resonance (NMR) spectroscopy is a key platform defining metabolic signatures in intact serum/plasma. In the present study, we used NMR-based analysis of macromolecules free-serum to accurately characterize the altered metabolic pathways of GDM and assessing their similarities to DM2. Our findings could contribute to the understanding of the pathophysiology of GDM and help in the identification of metabolomic markers of the disease.

METHODS

Sixty-two women with GDM matched with seventy-seven women without GDM (control group). ¹H NMR serum spectra were acquired on an 11.7 T Bruker Avance DRX NMR spectrometer.

RESULTS

We identified 55 metabolites in both groups, 25 of which were significantly altered in the GDM group. GDM group showed elevated levels of ketone bodies, 2-hydroxybutyrate and of some metabolic intermediates of branched-chain amino acids (BCAAs) and significantly lower levels of metabolites of one-carbon metabolism, energy production, purine metabolism, certain amino acids, 3-methyl-2-oxovalerate, ornithine, 2-aminobutyrate, taurine and trimethylamine N-oxide.

CONCLUSION

Metabolic pathways affected in GDM were beta-oxidation, ketone bodies metabolism, one-carbon metabolism, arginine and ornithine metabolism likewise in DM2, whereas BCAAs catabolism and aromatic amino acids metabolism were affected, but otherwise than in DM2.

Metabolomics and biomarkers in ocular matrix: beyond ocular diseases

According to the recent report, there are 870 million people suffer from ocular diseases worldwide. The present approaches for diagnosis are morphological examination, imaging examination and immunological examination, regrettably, they lack of sensitivity and difficult to make a definite diagnosis in the early stage. Systemic biology as an effective method has been used in clinical diagnosis and treatment for diseases, especially metabolomics which is more attractive with high sensitivity and accuracy. Although previous researches had been confirmed that endogenous metabolites in the ocular matrix play a crucial role in the progress of diseases related diseases, the standard protocols and systematic summary about the biomarker researches based on ocular matrix has not been established. This review article highlights the pretreatment for ocular matrix and the new biomarkers expressed by the eye diseases, expected to promote the application of biomarkers in the diagnosis and treatment of eye diseases.

Revisiting amino acids and peptides as anti-glycation agents

The importance of controlling or preventing protein glycation cannot be overstated and is of prime importance in the treatment of diabetes and associated complications including Alzheimer's disease, cataracts, atherosclerosis, kidney aliments among others. In this respect, simple molecules such as amino acids and peptides hold much promise both in terms of ease and scale-up of synthesis as well as in relation to negligible/low associated toxicity. In view of this, a comprehensive account of literature reports is presented, that documents the anti-glycation activity of natural and non-natural amino acids and peptides. This review also discusses the chemical reactions involved in glycation and the formation of advanced glycation end-products (AGEs) and possible/probable intervention sites and mechanism of action of the reported amino acids/peptides. This aspect of amino acids/peptides adds to their growing importance in medicinal and therapeutic applications.

Ameliorative effects of taurine against diabetes: a review

Diets in rats and humans have shown promising results. Taurine improved glucagon activity, promoted glycemic stability, modified glucose levels, successfully addressed hyperglycemia via advanced glycation end-product control, improved insulin secretion and had a beneficial effect on insulin resistance. Taurine treatment performed well against oxidative stress in brain, increased the secretion of required hormones and protected against neuropathy, retinopathy and nephropathy in diabetes compared with the control. Taurine has been observed to be effective in treatments against diabetic hepatotoxicity, vascular problems and heart injury in diabetes. Taurine was shown to be effective against oxidative stress. The mechanism of action of taurine cannot be explained by one pathway, as it has many effects. Several of the pathways are the advanced glycation end-product pathway, PI3-kinase/AKT pathway and mitochondrial apoptosis pathway. The worldwide threat of diabetes underscores the urgent need for novel therapeutic measures against this disorder. Taurine (2-aminoethane sulfonic acid) is a natural compound that has been studied in diabetes and diabetes-induced complications.

An exploratory LC-MS-based metabolomics study reveals differences in aqueous humor composition between diabetic and non-diabetic patients with cataract

Cataract is the leading cause of blindness worldwide. Epidemiological studies revealed up to a fivefold increased prevalence of cataracts in diabetic subjects. Metabolomics is nowadays frequently implemented to understand pathophysiological processes responsible for disease occurrence and progression. It has also been used recently to study the metabolic composition of aqueous humor (AH). AH is a transparent fluid which fills the anterior and posterior chambers of the eye. It supplies nutrients and removes metabolic waste from avascular tissues in the eye. The aim of this study was to use metabolomics to compare the AH of diabetic and non-diabetic patients undergoing cataract surgery. Several antioxidants (methyltetrahydrofolic acid, taurine, niacinamide, xanthine, and uric acid) were found decreased (-22 to -61%, p-value 0.05-0.003) in AH of diabetics. Also amino acids (AA) and derivatives were found decreased (-21 to -36%, p-value 0.05-0.01) while glycosylated AA increased (+75-98%, p-value 0.03-0.009) in this group of patients. Metformin was detected in AH of people taking this drug. To our knowledge, this is the first metabolomics study aiming to assess differences in AH composition between diabetic and non-diabetic patients with cataract. An increased oxidative stress and perturbations in amino acid metabolism in AH may be responsible for earlier cataract onset in diabetic patients.

Maillard reactions in hyperthermophilic archaea: implications for better understanding of non-enzymatic glycation in biology

Maillard reactions are an unavoidable feature of life that appear to be damaging to cell and organisms. Consequently, all living systems must have ways to protect themselves against this process. As of 2012, several such defense mechanisms have been identified. They are all enzymatic and were found in mesophilic organisms. To date, no systematic study of Maillard reactions and the relevant defense mechanisms has been conducted in thermophiles (50°C-80°C) or hyperthermophiles (80°C-120°C). This is surprisingly because Maillard reactions become significantly faster and potent with increasing temperatures. This review examines this neglected issue in two well-defined sets of hyperthermophiles. My analysis suggests that hyperthermophiles cope with glycation stress by several mechanisms: • Absence of glycation-prone head groups (such as ethanoalamine) from hyperthermophilic phospholipids • Protection of reactive carbohydrates and labile metabolic intermediates by substrate channeling. • Conversion of excess reactive sugars such as glucose to non-reactive compounds including trehalose, di-myo-inositol-phosphate and mannosylglycerate. • Detoxification of methylglyoxal and other ketoaldehydes by conversion to inert products through a variety of reductases and dehydrogenases. • Scavenging of the remaining carbonyls by nucleophilic amines, including a variety of novel polyamines. Disruption of the Maillard process at its early stages, rather than repair of damage caused by it at later stages, appears to be the preferred strategy in the organisms examined. The most unique among these mechanisms appears to be a polyamine-based scavenging system. Undertaking research of the Maillard process in hyperthermophiles is important in its own right and is also likely to provide new insights for the control of these reactions in humans, especially in diseases such as diabetes mellitus.

Traditional reactive carbonyl scavengers do not prevent the carbonylation of brain proteins induced by acute glutathione depletion

This study investigated the effect of reactive carbonyl species (RCS)-trapping agents on the formation of protein carbonyls during depletion of brain glutathione (GSH). To this end, rat brain slices were incubated with the GSH-depletor diethyl maleate in the absence or presence of chemically different RCS scavengers (hydralazine, methoxylamine, aminoguanidine, pyridoxamine, carnosine, taurine and z-histidine hydrazide). Despite their strong reactivity towards the most common RCS, none of the scavengers tested, with the exception of hydralazine, prevented protein carbonylation. These findings suggest that the majority of protein-associated carbonyl groups in this oxidative stress paradigm do not derive from stable lipid peroxidation products like malondialdehyde (MDA), acrolein and 4-hydroxynonenal (4-HNE). This conclusion was confirmed by the observation that the amount of MDA-, acrolein- and 4-HNE-protein adducts does not increase upon GSH depletion. Additional studies revealed that the efficacy of hydralazine at preventing carbonylation was due to its ability to reduce oxidative stress, most likely by inhibiting mitochondrial production of superoxide and/or by scavenging lipid free radicals.

Cytoprotective role of mitogen-activated protein kinase phosphatase-1 in light-damaged human retinal pigment epithelial cells

The role of the mitogen-activated protein (MAP) kinase phosphatases (MKPs) in light-damaged cells is unclear. Therefore we investigated the involvement of MKP-1 in the regulation of apoptosis and cell survival mediated by MAP kinase pathways in light-damaged human retinal pigment epithelial cells (ARPE-19). Light dose-dependent changes in the expression of MKP-1 and in the phosphorylation status of the MAP kinases, c-Jun-N-terminal kinase (JNK) and p38 were demonstrated. Low light doses up to 2 J cm(-2) led to an upregulation of MKP-1 which resulted in the prevention of cell death by inactivating JNK kinase. However, higher light doses (> or =3 J cm(-2)) significantly reduced MKP-1 protein expression and subsequently led to an increased JNK kinase activity followed by a significant increase in cell death. JNK kinase inactivation by the JNK inhibitor SP600125 significantly reduced light-induced cell death, suggesting that the cytoprotective properties of MKP-1 are mediated mainly by the JNK MAP kinase pathway. Physiological concentrations of ascorbic acid or taurine were seen to prevent apoptosis and cell death in light-damaged ARPE-19 cells by reducing oxidative stress within cells, thus maintaining MKP-1 at high levels, leading to an inactivation of the JNK kinase pathway which resulted in an increased cell viability.

HNE-dependent molecular damage in diabetic nephropathy and its possible prevention by N-acetyl-cysteine and oxerutin

Accumulation of Advanced Lipoxidation End-products (ALE), such as MDA- and HNE-protein adducts, and Advanced Glycation End-products, such as carboxymethyl-lysine (CML), are probably involved in the development of diabetic nephropathy. In this study the effect of some antioxidant treatments (oxerutin, N-acetylcysteine, taurine and N-acetylcysteine+taurine) on kidney lipoxidative damage has been evaluated by immunohistochemistry in streptozotocined rats. Diabetic rats showed marked glomerular positivity for ALE, while the samples from Control rats were negative. All treatments except taurine were able to protect the glomeruli from ALE accumulation; the failure of taurine may be due to residual oxidative properties of its derivatives. These data are consistent with those of our previous study, which showed that all the antioxidants used except taurine protected the glomeruli from diabetes-induced enlargement, increased apoptotic rate, decreased cell density and CML accumulation. These data attest to a role of glycoxidative and lipoxidative damage in diabetes-dependent damage of the kidney, and indicate that specific antioxidants can prevent or attenuate diabetic nephropathy.

Is taurine beneficial in reducing risk factors for diabetes mellitus?

Taurine is a semiessential amino acid, and its deficiency is involved in retinal and cardiac degenerations. In recent years, it was found that diabetes mellitus (DM) is associated with taurine, and many in vivo experimental studies showed that taurine administration is able to reduce the alterations induced by DM in the retina, lens, and peripheral nerve, although its effects on diabetic kidney are dubious. Interestingly, long-term taurine supplementation reduces the mortality rate in diabetic rats. The mechanisms by which taurine exerts beneficial effects in DM are discussed below. Recently, it has been suggested that taurine deficiency may alter the endocrine pancreas "fetal programming," increasing the risk of insulin resistance in adult life. The bulk of experimental data suggests that taurine administration could be useful in the treatment of type 1 DM and in the prevention of insulin resistance.

Age-related retinal degeneration in animal models of aging: possible involvement of taurine deficiency and oxidative stress

There is strong evidence that the retina degenerates with age. Electroretinogram deficits and photoreceptor cell death and structural abnormalities have been observed in both animal and human studies of aging. The mechanism behind this phenomenon is a very interesting area for scientific and medical study. Current data support the link between retinal degeneration and increased oxidative stress. Taurine is a free amino acid found in high millimolar concentrations in the retina, and age-related deficiency in retinal levels of taurine may contribute to the retinal degeneration associated with age. Taurine acts as an antioxidant and taurine replenishment is known to alleviate oxidative stress in the retina. Thus taurine supplementation may be useful in the treatment of age-related retinal dysfunction.

Stimulation of glucose utilization and inhibition of protein glycation and AGE products by taurine

AIM

Pathological effects of the process of non-enzymatic glycation of proteins are reflected in chronic complications of diabetes mellitus. We investigated the antiglycating effect of taurine in high fructose fed rats in vivo and the inhibiting potency of taurine in the process of in vitro glycation. Additionally, we investigated whether taurine enhances glucose utilization in the rat diaphragm.

METHODS

Rats fed a high fructose diet (60% total calories) were provided 2% taurine solution for 30 days. The effects of taurine on plasma glucose, fructosamine, protein glycation and glycosylated haemoglobin in high fructose rats were determined. For in vitro glycation a mixture of 25 mm glucose and 25 mm fructose was used as glycating agent, bovine serum albumin as the model protein and taurine as the inhibitor. Incubations were carried out in a constant temperature bath at 37 degrees C for 3-30 days. Amadori products and advanced glycation end products (AGEs) formed were measured. In vitro utilization of glucose was carried out in the rat diaphragm in the presence and absence of insulin in which taurine was used as an additive.

RESULTS

The contents of glucose, glycated protein, glycosylated haemoglobin and fructosamine were significantly lowered by taurine treatment to high fructose rats. Taurine prevented in vitro glycation and the accumulation of AGEs. Furthermore, taurine enhanced glucose utilization in the rat diaphragm. This effect was additive to that of insulin and did not interfere with the action of insulin.

CONCLUSIONS

These results underline the potential use of taurine as a therapeutic supplement for the prevention of diabetic pathology.

Inhibition of ascorbic acid-induced modifications in lens proteins by peptides

The effects of three dipeptides L-phenylalanyl-glybine, glycyl-L-phenylalanine,and aspartame (L-aspartyl-L-phenylalanine, methyl ester) as inhibitors of the ascorbic acid-induced modifications in lens proteins were studied. Their efficiency was compared to that of two known inhibitors--aminoguanidine and carnosine. The tested dipeptides diminished protein carbonyl content by 32-58% and most moderated the formation of chromophores, as measured by the absorbency at 325 nm of the glycated proteins. The appearance of non-tryptophan fluorescence (excitation 340 nm/emission 410 nm) was observed for proteins glycated with ascorbic acid. All of the dipeptides examined, as well as aminoguanidine, decreased this glycation-related fluorescence. The potential inhibitors prevented the intensive formation of very high molecular weight aggregates. A competitive mechanism of their inhibitory effect was proposed, based on the reactivity of individual substances toward ascorbic acid. These findings indicate that they have a potential for use as alternatives for aminoguanidine as an anti-glycation agent.

Comparative trial of N-acetyl-cysteine, taurine, and oxerutin on skin and kidney damage in long-term experimental diabetes

This study analyzes the effect of chronic treatment with different antioxidants (N-acetyl-cysteine [NAC], taurine, a combination of NAC and taurine, and oxerutin) on long-term experimental diabetes induced by streptozotocin in rats. Glycoxidative damage was evaluated in the skin; glomerular structural changes were studied with morphometry and immunohistochemistry. Oxerutin treatment and the combined NAC plus taurine treatment resulted in reduced accumulation of collagen-linked fluorescence in skin in comparison with untreated diabetic rats. All treatments except taurine reduced glomerular accumulation of N(epsilon)-(carboxymethyl)lysine and protected against the increase in glomerular volume typical of diabetes; furthermore, the apoptosis rate was significantly decreased and the glomerular cell density was better preserved. Glycoxidative markers in the skin turned out to be good indicators of the glomerular condition. The findings that emerged from our study support the hypothesis that glomerular damage in diabetes can be prevented or at least attenuated by supplementation with specific antioxidants. Treatment with oxerutin and combined treatment with NAC plus taurine gave the most encouraging results, whereas the results of taurine-only treatment were either negligible or negative and therefore suggest caution in the use of this molecule in single-drug treatment courses.

The role of taurine in diabetes and the development of diabetic complications

The ubiquitously found beta-amino acid taurine has several physiological functions, e.g. in bile acid formation, as an osmolyte by cell volume regulation, in the heart, in the retina, in the formation of N-chlorotaurine by reaction with hypochlorous acid in leucocytes, and possibly for intracellular scavenging of carbonyl groups. Some animals, such as the cat and the C57BL/6 mouse, have disturbances in taurine homeostasis. The C57BL/6 mouse strain is widely used in diabetic and atherosclerotic animal models. In diabetes, the high extracellular levels of glucose disturb the cellular osmoregulation and sorbitol is formed intracellularly due to the intracellular polyol pathway, which is suspected to be one of the key processes in the development of diabetic late complications and associated cellular dysfunctions. Intracellular accumulation of sorbitol is most likely to cause depletion of other intracellular compounds including osmolytes such as myo-inositol and taurine. When considering the clinical complications in diabetes, several links can be established between altered taurine metabolism and the development of cellular dysfunctions in diabetes which cause the clinical complications observed in diabetes, e.g. retinopathy, neuropathy, nephropathy, cardiomyopathy, platelet aggregation, endothelial dysfunction and atherosclerosis. Possible therapeutic perspectives could be a supplementation with taurine and other osmolytes and low-molecular compounds, perhaps in a combinational therapy with aldose reductase inhibitors.

Inhibitors of advanced glycation end product-associated protein cross-linking

The reaction of lens proteins with sugars over time results in the formation of protein-bound advanced glycation end products (AGEs). The most damaging element of AGE formation may be the synthesis of protein-protein cross-links in long-lived proteins, such as collagen or lens crystallins. A quantitative cross-linking assay, involving the sugar-dependent incorporation of [U-(14)C]lysine into protein, was employed to determine the efficacy of a variety of potential cross-linking inhibitors. Reaction mixtures contained 5.0 mM L-threose, 2.5 microCi [(14)C]lysine (1.0 mCi/mmole), 5.0 mg/ml bovine lens proteins, 0-10 mM inhibitor and 1.0 mM DTPA in 100 mM phosphate buffer, pH 7.0. Of 17 potential inhibitors tested, 11 showed 50% inhibition or less at 10 mM. The dicarbonyl-reactive compounds 2-aminoguanidine, semicarbazide and o-phenylenediamine inhibited 50% at 2.0 mM, whereas 10 mM dimethylguanidine had no effect. Several amino acids failed to compete effectively with [(14)C]lysine in the cross-linking assay; however, cysteine inhibited 50% at 1.0 mM. This was likely due to the sulfhydryl group of cysteine, because 3-mercaptopropionic acid and reduced glutathione exhibited similar activity. Sodium metabisulfite had the highest activity, inhibiting 50% at only 0.1-0.2 mM. Protein dimer formation, as determined by SDS-PAGE, was inhibited in a quantitatively similar manner. The dicarbonyl-reactive inhibitors and the sulfur-containing compounds produced similar inhibition curves for [(14)C]lysine incorporation over a 3 week assay with 250 mM glucose. A much lesser effect was observed on either the incorporation of [(14)C]glucose, or on fluorophore formation (360/420 nm), suggesting that non-cross-link fluorophores were also formed. The inhibitor data were consistent with cross-linking by a dicarbonyl intermediate. This was supported by the fact that the inhibitors were uniformly less effective when the 5.0 mM threose was replaced by either 3.0 mM 3-deoxythreosone or 3.0 mM threosone.

Non-enzymatic glycation of lens proteins and haemoglobin-inhibition by pyruvate: an in-vivo study

AIM

Previous studies have demonstrated that pyruvate can prevent protein glycation and oxidative stress under in-vitro conditions. The aim of this study was to examine the in-vivo effectiveness of this metabolite against glycation of lens crystallins and haemoglobin in galactosemic rats.

METHODS

Sprague-Dawley rats were maintained on a 30% or 50% galactose-containing diet in the absence or presence of 2% or 5% pyruvate in food and water, respectively. The animals were killed subsequently and the extent of glycation of lens crystallins and haemoglobin was determined using an affinity column chromatograpic technique.

RESULTS

Maintenance of rats on the high galactose diet resulted in a significant increase in glycation of both the proteins. The increase was faster and more substantial in the animals maintained on the 50% galactose diet than that in the animals fed a 30% galactose diet. The increase in the latter was also very significant. Supplementation with pyruvate inhibited the process.

CONCLUSION

The inhibition is attributable to a competitive binding of pyruvate to the protein NH2 groups as well as to the antioxidant effect of the compound. The studies therefore suggest that this and other alpha-keto-acids may be physiologically useful in minimizing glycation and oxidative stress induced tissue pathology by the hyperglycaemic conditions, such as diabetes and galactosemia. The results are also considered pharmacologically significant.

Oxidative stress to rat lens in vitro: protection by taurine

The concentration of taurine is high in the lens. However, its function therein remains unknown. Studies from other tissues suggest that in addition to several other modes of action, it acts as an antioxidant. We therefore hypothesize that taurine may be a part of the antioxidant defense mechanisms involved in protecting the lens against oxidative stress and consequent cataract formation. In these studies, the protective effect of taurine was examined using lens culture system with menadione as an oxidant. Inclusion of this compound in the incubation medium was found to have several adverse effects on the lens, such as a decrease in its ability to accumulate rubidium against a concentration gradient and fall in the levels of glutathione, ATP and an increase in water insoluble proteins. All these deleterious effects were attenuated significantly by addition of physiological amounts of taurine to the menadione-containing medium.